I am trying to install The Sims 4 but when I put in the code a message keeps
saying that this code has already been used. I had it installed on PC and laptop
which I also uninstalled some time ago. Why is it doing this and what should I do?

After you download it, it should be in your downloads folder. Drag or cut that into your Sims mods folder by going to Documents>Electronic Arts>Sims 4>Mods. If there isn’t a Mods folder, you can just make one